The present invention relates to a DC/DC converter which has a high step-up/step-down ratio and operates in both directions by a regenerative operation or the like, and more particularly, to a miniaturization technique.
Recently, a 42V system and a 14V system are used as electric parts of automobiles. A DC power supply (first battery) of the 42V system is charged with power provided by a power generator, charges a DC power supply (second battery) of the 14V system through the DC/DC converter, and supplies power to electric parts of the 14V system. When the second battery has excessive electric power, the power is supplied to the first battery from the second battery through the DC/DC converter.
A DC/DC converter for the 14V system for driving the electric parts of the 42V system has high output current, and since the output current and current passing through a switching device are substantially the same, the switching device of high current is connected to the DC/DC converter in parallel.
FIG. 1 is a circuit diagram of a conventional DC/DC converter. The DC/DC converter shown in FIG. 1 has a DC power supply Vdc1 (e.g., 42V) as a first DC power supply and a DC power supply Vdc2 (e.g., 14V) as a second DC power supply. This DC/DC converter is a bidirectional converter that steps up and steps down voltage by supplying power in two directions between the DC power supply Vdc1 and the DC power supply Vdc2.
In FIG. 1, a series circuit having a switch Q1 including an MOSFET and a switch Q2 including an MOSFET is connected to both ends of the DC power supply Vdc1. A series circuit of a reactor L1, a current detection circuit 107, and the DC power supply Vdc2 is connected to both ends of the switch Q2.
The current detection circuit 107 detects current passing through a reactor L1, and outputs the current to a step-up/step-down switching circuit 108. The step-up/step-down switching circuit 108 detects voltage of the DC power supply Vdc1 and voltage of the DC power supply Vdc2, and outputs a switching signal for switching a step-up/step-down operation to a control circuit 110 according to the voltage of the DC power supply Vdc2, the voltage of the DC power supply Vdc1, and polarity of the current detected by the current detection circuit 107.
The control circuit 1100N/OFF controls the switch Q1 and the switch Q2 based on a switching signal from the step-up/step-down switching circuit 108, thereby controlling the voltage step-down operation (control of charging) from the DC power supply Vdc1 to the DC power supply Vdc2, and controlling the voltage step-up operation (control of a discharging or regenerative operation) from the DC power supply Vdc2 to the DC power supply Vdc1.
An operation of the conventional DC/DC converter thus configured will be explained with reference to a timing chart of each signal at the time of a voltage step-down operation shown in FIG. 2 and a timing chart of each signal at the time of a voltage step-up operation shown in FIG. 3.
In FIGS. 2 and 3, a reference symbol Q1v represents voltage between a drain and a source of the switch Q1, Q1i represents drain current of the switch Q1, Q2v represents voltage between a drain and a source of the switch Q2, Q2i represents drain current of the switch Q2, and L1i represents current passing through the reactor L1.
The operation at the time of voltage step-down operation shown in FIG. 2, that is, an operation for lowering voltage from 42V to 14V by supplying power of the DC power supply Vdc1 to the DC power supply Vdc2 will be explained. At time t0, if the switch Q2 is turned OFF and the switch Q1 is turned ON, current passes through a path extending along Vdc1 plus terminal, Q1, L1, Vdc2, and Vdc1 minus terminal, and the DC power supply Vdc2 is charged with power. Thus, the current Q1i of the switch Q1 is increased straightly with respect to time. At the same time, the current L1i of the reactor L1 is also increased straightly with respect to time.
In duration from time t11 to time t12, if the switch Q1 is turned OFF and the switch Q2 is turned ON, the current Q1i of the switch Q1 abruptly becomes a zero value, and the current Q2i of the switch Q2 abruptly increases to a given value and is then lowered straightly. At that time, current passes through a path extending along L1, Vdc2, Q2, and L1 by energy stored in the reactor L1. Therefore, the DC power supply Vdc2 is charged with electricity. The current L1i of the reactor L1 is also lowered from a peak value such as to draw an inclination corresponding to a difference value between input voltage and output voltage. The operation after time t12 is the same as that from time t0 to time t12. Current Q1i of the switch Q1, current Q2i of the switch Q2, and current L1i of the reactor L1 are in a positive direction.
An operation at the time of the voltage step-up operation shown in FIG. 3, that is, a voltage step-up operation from 14V to 42V (a regenerative operation) by supplying power of the DC power supply Vdc2 to the DC power supply Vdc1 will be explained. At time t11, if the switch Q1 is turned OFF and the switch Q2 is turned ON, current passes through a path extending along Vdc2 plus terminal, L1, Q2, and Vdc2 minus terminal. Thus, current Q2i of the switch Q2 increases straightly. At the same time, current L1i of the reactor L1 also increases straightly.
Induration of time t12 to time t13, if the switch Q2 is turned OFF and the switch Q1 is turned ON, current Q2i of the switch Q2 abruptly becomes a zero value, and the current Q1i of the switch Q1 abruptly increases to a given value and then is lowered straightly. At that time, current passes through a path extending along Vdc2 plus terminal, L1, Q1, Vdc1, and Vdc2 minus terminal by energy stored in the reactor L1, and, the DC power supply Vdc1 is charged with electricity. The current L1i of the reactor L1 is lowered. The operation after time t13 is the same as that from time t11 to time t13. Current Q1i of the switch Q1, current Q2i of the switch Q2, and current L1i of the reactor L1 are in a negative direction.